The present invention relates to a marine exploration apparatus and more specifically to a seabed surveying apparatus which combines sidescan imaging techniques with hydrophonic techniques to provide a representation of topographic features superimposed on contour lines.
One typical method currently employed in the survey of seabed is sidescan imaging techniques. Sidescan sonar images are a mapping of a visible representation of the strength of acoustic back scattering from the sea floor onto a two-dimensional image medium. Scanning takes place in two directions, i.e. along the survey track and perpendicular to it. Cross-track scanning is achieved by emission of a sound wave through the water and the reception of echoes from successively greater ranges. Along-track scanning is achieved by physical translation of the transducer. In this direction, the object is sampled by a sequence of discrete pulse emission. The result is that the picture elements take on a larger and larger aspect ratio parallel to the track, leading to the impression that all far-range bottom targets have linear structure parallel to the track. One disadvantage of the sidescan imaging is that it lacks bathymetric information and at close ranges the bottom is often undersampled with the possibility of aliasing or missing features.
Another method employed in the survey of seabed is a multi-beam echo-sounder which transmits a narrow fan beam as the surveillance ship moves along a track so that the area of sea floor covered with it corresponds to a rectangular area crossing the track. An array of transducers are responsive to echoes returning from a plurality of successively arranged rectangular areas each crossing the area covered at emission. A plurality of square zones, or pixels, result from the combination of the two sets of intersecting areas. Hydrophonic record is made to plot contour lines with a high degree of resolution at close ranges. However, the resolution degrades at far ranges. Furthermore, due to the inherent discreteness of the data obtained, it is desirable to provide means by which interpolation can be made between successive contour lines.